This invention relates in general to an apparatus for increasing the rate of heat transfer between a heated surface and a liquid coolant, and finds particular utility in cooling of semiconductor elements and circuits. Additionally, the invention relates to a housing or support for semiconductor elements and circuits having surfaces which are particularly adapted to increase the rate of heat transfer between the surfaces and the liquid coolant in contact therewith.
In modern high density semiconductor packaging technology it is necessary to transfer heat from the surface of a heated semiconductor element to a heat sink at a rate sufficiently high to limit the unavoidable increase in the temperature of the semiconductor material. One such technique for transferring heat is to immerse the semiconductor devices in a liquid coolant which is enclosed by a suitable chamber usually provided with suitable cooling fins. In operation the heat generated in the semiconductor devices causes the liquid coolant to boil at the surface of the chip. The heat generated by the semiconductor is transferred to the liquid coolant when the liquid is vaporized. A portion of the heat is absorbed as the latent heat of vaporization of the liquid, the remainder being absorbed by the convection of the liquid at the chip surface. The motion of the vapor bubbles leaving the surface of the chip breaks up the stagnant boundary layer of fluid at the chip surface, greatly enhancing the convective heat flow. The vapor is subsequently condensed on the relatively cold finned sidewall of the chamber containing the liquid. Typical integrated circuit packages of this type are illustrated and disclosed in U.S. Pat. No. 3,741,292 and U.S. Pat. No. 3,851,221.
A significant problem is encountered in liquid cooled semiconductor packages in promoting nucleate boiling. The liquid coolant must be very pure since it comes into contact with the metallurgy on the substrate supporting the devices and also the device contacts. Any foreign material contained in the liquid coolant may cause contamination problems in the package. This requirement precludes the presence of material in the liquid which might otherwise serve as centers to promote nucleate boiling. Also the semiconductor devices mounted in the package do not ordinarily have the type of surface exposed to the liquid which will promote nucleate boiling. The silicon surface is normally highly polished and therefore devoid of centers which would promote nucleate boiling. Still further the cooling fluids normally used in such packages, typically fluorocarbons, wet the silicon and therefore inhibit any imperfections from becoming nucleate boiling centers which might otherwise function as such in other fluids. Various techniques have been suggested for forming nucleate boiling centers on the silicon devices such as sand blasting or etching, as suggested in IBM Technical Disclosure Bulletin Vol. 19, No. 3, Aug. 1976, Page 937. However, such techniques have not been satisfactory for a Si-fluorocarbon system since an insufficient number of nucleate boiling centers are formed. Another approach has been suggested, namely, to provide a heater consisting of a high resistance wire connected to a suitable source of current which will initiate and maintain a boiling action. However, this approach is also not satisfactory since (1) additional heat is being introduced into the package which is undesirable; and (2) the boiling should preferably occur on a surface on the semiconductor device. While the aforementioned heater element will prevent flash boiling and destruction of the package, as when the coolant becomes highly super heated, it does not provide the type of cooling necessary in a high density semiconductor package.